Photoreduction of ortho-benzoquinones in the presence of para-substituted N,N-dimethylanilines

Article abstract of DOI:10.1007/BF02495150

Photoreduction of o-benzoquinones irradiated at the wavelengths λ_max ≈ 400 and 600 nm corresponding to the S(π→π*) and S(n→π*) electron transitions in the >C=O groups, respectively, in the presence of N,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (k_H) of o-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcited o-quinone molecule (ΔG_e.t). The ΔG_e.t values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state of o-quinones, the reduction energies of o-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔG_e.t ≈ 0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex.